Electrode for secondary battery, preparation thereof, and secondary battery and cable-type secondary battery comprising the same

ABSTRACT

The present disclosure provides a sheet-form electrode for a secondary battery, comprising a current collector; an electrode active material layer formed on one surface of the current collector; a conductive layer formed on the electrode active material layer and comprising a conductive material and a binder; and a first porous supporting layer formed on the conductive layer. The sheet-form electrode for a secondary battery according to the present disclosure has supporting layers on at least one surfaces thereof to exhibit surprisingly improved flexibility and prevent the release of the electrode active material layer from a current collector even if intense external forces are applied to the electrode, thereby preventing the decrease of battery capacity and improving the cycle life characteristic of the battery.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No.PCT/KR2014/004046 filed on May 7, 2014, which claims priority under 35USC 119(a) to Korean Patent Application No. 10-2013-0051564 filed in theRepublic of Korea on May 7, 2013, and Korean Patent Application No.10-2014-0054278 filed in the Republic of Korea on May 7, 2014, thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates to an electrode for a secondary battery,more specifically to an electrode for a secondary battery which can beprevented from the release of an electrode active material layer andhave improved flexibility, a method of preparing the electrode, and asecondary battery and a cable-type secondary battery comprising theelectrode.

2. Description of the Related Art

Secondary batteries are devices capable of storing energy in chemicalform and of converting into electrical energy to generate electricitywhen needed. The secondary batteries are also referred to asrechargeable batteries because they can be recharged repeatedly. Commonsecondary batteries include lead accumulators, NiCd batteries, NiMHaccumulators, Li-ion batteries, Li-ion polymer batteries, and the like.When compared with disposable primary batteries, not only are thesecondary batteries more economically efficient, they are also moreenvironmentally friendly.

Secondary batteries are currently used in applications requiring lowelectric power, for example, equipment to start vehicles, mobiledevices, tools, uninterruptible power supplies, and the like. Recently,as the development of wireless communication technologies has beenleading to the popularization of mobile devices and even to themobilization of many kinds of conventional devices, the demand forsecondary batteries has been dramatically increasing. Secondarybatteries are also used in environmentally friendly next-generationvehicles such as hybrid vehicles and electric vehicles to reduce thecosts and weight and to increase the service life of the vehicles.

Generally, secondary batteries have a cylindrical, prismatic, or pouchshape. This is associated with a fabrication process of the secondarybatteries in which an electrode assembly composed of an anode, acathode, and a separator is mounted in a cylindrical or prismatic metalcasing or a pouch-shaped casing of an aluminum laminate sheet, and inwhich the casing is filled with electrolyte. Because a predeterminedmounting space for the electrode assembly is necessary in this process,the cylindrical, prismatic or pouch shape of the secondary batteries isa limitation in developing various shapes of mobile devices.Accordingly, there is a need for secondary batteries of a new structurethat are easily adaptable in shape.

To fulfill this need, suggestions have been made to develop cable-typebatteries having a very high ratio of length to cross-sectionaldiameter. The cable-type batteries are easy in shape variation, whilebeing subject to stress due to external force for the shape variation.Also, the electrode active material layer of cable-type batteries may bereleased by rapid volume expansion during charging and dischargingprocesses. From these reasons, the capacity of the batteries may bereduced and the cycle life characteristics thereof may be deteriorated.

Such a problem may be solved in a certain degree by increasing theamount of a binder used in the electrode active material layer toprovide flexibility during bending or twisting. However, the increase ofa binder amount in the electrode active material layer causes anelectrode resistance rise to deteriorate battery performances. Also,when intense external forces are applied, for example, in the case thatelectrodes are completely folded, the release of the electrode activematerial layer cannot be prevented even though the amount of a binderbecomes increased. Therefore, this method is insufficient to solve suchproblems.

SUMMARY OF THE DISCLOSURE

The present disclosure is designed to solve the problems of the relatedart, and therefore the present disclosure is directed to providing anelectrode for a secondary battery which can be mitigated from crackgeneration in an electrode active material layer by external forces, andalso can be prevented from the release of the electrode active materiallayer from a current collector even if severe cracks are present, amethod of preparing the electrode, and a secondary battery and acable-type secondary battery comprising the electrode.

In accordance with one aspect of the present disclosure, there isprovided a sheet-form electrode for a secondary battery, comprising acurrent collector; an electrode active material layer formed on onesurface of the current collector; a conductive layer formed on theelectrode active material layer and comprising a conductive material anda binder; and a first porous supporting layer formed on the conductivelayer.

The current collector may be made of stainless steel, aluminum, nickel,titanium, sintered carbon, or copper; stainless steel treated withcarbon, nickel, titanium or silver on the surface thereof; analuminum-cadmium alloy; a non-conductive polymer treated with aconductive material on the surface thereof; a conductive polymer; ametal paste comprising metal powders of Ni, Al, Au, Ag, Pd/Ag, Cr, Ta,Cu, Ba or ITO; or a carbon paste comprising carbon powders of graphite,carbon black or carbon nanotube.

Also, the current collector may be in the form of a mesh.

In addition, the current collector may further comprise a primer coatinglayer consisting of a conductive material and a binder.

The conductive material may comprise any one selected from the groupconsisting of carbon black, acetylene black, ketjen black, carbon fiber,carbon nanotube, graphene and a mixture thereof.

The binder may be selected from the group consisting of polyvinylidenefluoride (PVDF), polyvinylidene fluoride-co-hexafluoro propylene,polyvinylidene fluoride-co-trichloroethylene, polybutyl acrylate,polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone,polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide,polyarylate, cellulose acetate, cellulose acetate butyrate, celluloseacetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol,cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methylcellulose, styrene-butadiene rubber, acrylonitrile-styrene-butadienecopolymer, polyimide and a mixture thereof.

Further, the current collector may have a plurality of recesses whichare continuously patterned or intermittently patterned, on at least onesurface thereof.

Meanwhile, the first supporting layer may be a mesh-form porous membraneor a non-woven fabric.

The first supporting layer may be made of any one selected from thegroup consisting of high-density polyethylene, low-density polyethylene,linear low-density polyethylene, ultra-high molecular weightpolyethylene, polypropylene, polyethylene terephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate,polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide,polyphenylene sulfide, polyethylene naphthalate, and a mixture thereof.

Also, the first supporting layer may further comprise a conductivematerial-coating layer having a conductive material and a binderthereon.

In the conductive material-coating layer, the conductive material andthe binder may be present in a weight ratio of 80:20 to 99:1.

Meanwhile, the second supporting layer may be a polymer film which maybe made of any one selected from the group consisting of polyolefin,polyester, polyimide, polyamide and a mixture thereof.

The conductive layer may be formed from a mixture of the conductivematerial and the binder in a weight ratio of 1:10 to 8:10.

Also, the conductive layer may be a porous layer having a pore size of0.01 to 5 and a porosity of 5 to 70%.

The conductive material may comprise any one selected from the groupconsisting of carbon black, acetylene black, ketjen black, carbon fiber,carbon nanotube, graphene and a mixture thereof.

The binder may be selected from the group consisting of polyvinylidenefluoride (PVDF), polyvinylidene fluoride-co-hexafluoro propylene,polyvinylidene fluoride-co-trichloroethylene, polybutyl acrylate,polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone,polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide,polyarylate, cellulose acetate, cellulose acetate butyrate, celluloseacetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol,cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methylcellulose, styrene-butadiene rubber, acrylonitrile-styrene-butadienecopolymer, polyimide and a mixture thereof.

In addition, the first supporting layer may further comprise a porouscoating layer comprising a mixture of inorganic particles and a binderpolymer thereon.

Meanwhile, the sheet-form electrode may further comprise a secondsupporting layer on another surface of the current collector.

The second supporting layer may be a polymer film which may be made ofany one selected from the group consisting of polyolefin, polyester,polyimide, polyamide and a mixture thereof.

Meanwhile, the inner electrode may be an anode or a cathode, and theouter electrode may be a cathode or an anode corresponding to the innerelectrode.

When the electrode for a secondary battery is used as an anode, theelectrode active material layer may comprise an active material selectedfrom the group consisting of natural graphite, artificial graphite, orcarbonaceous material; lithium-titanium complex oxide (LTO), and metals(Me) including Si, Sn, Li, Zn, Mg, Cd, Ce, Ni and Fe; alloys of themetals; an oxide (MeOx) of the metals; a complex of the metals andcarbon; and a mixture thereof, and when the electrode for a secondarybattery is used as a cathode, the electrode active material layer maycomprise an active material selected from the group consisting ofLiCoO₂, LiNiO₂, LiMn₂O₄, LiCoPO₄, LiFePO₄, LiNiMnCoO₂,LiNi_(1-x-y-z)Co_(x)M1_(y)M2_(z)O₂ (wherein M1 and M2 are eachindependently selected from the group consisting of Al, Ni, Co, Fe, Mn,V, Cr, Ti, W, Ta, Mg and Mo, and x, y and z are each independently anatomic fraction of oxide-forming elements, in which 0≦x<0.5, 0≦y<0.5,0≦z<0.5, and x+y+z≦1), and a mixture thereof.

In accordance with another aspect of the present disclosure, there isprovided a method of preparing a sheet-form electrode for a secondarybattery, comprising (S1) applying a slurry containing an electrodeactive material on one surface of the current collector, followed bydrying, to form an electrode active material layer; (S2) applying aslurry containing a conductive material and a binder on the electrodeactive material layer; (S3) forming a first porous supporting layer onthe applied slurry containing a conductive material and a binder; and(S4) compressing the resultant obtained in step (S3) to form aconductive layer which is adhered between the electrode active materiallayer and the first porous supporting layer to be integrated with eachother.

In the step of (S3), before the binder is cured, the first poroussupporting layer may be formed on the applied conductive materialslurry.

In the step of (S4), before the binder is cured, the resultant obtainedin step (S3) is compressed by a coating blade to form a conductivematerial layer which is adhered between the electrode active materiallayer and the first porous supporting layer to be integrated with eachother

Also, before the step of (S1) or after the step of (S4), a secondsupporting layer may be further formed on another surface of the currentcollector by compression.

Also, in accordance with yet another aspect of the present disclosure,there is provided a secondary battery comprising a cathode, an anode, aseparator interposed between the cathode and the anode, and anon-aqueous electrolyte solution, wherein at least one of the cathodeand the anode is the above-mentioned electrode for a secondary batteryaccording to the present disclosure.

In addition, in accordance with yet still another aspect of the presentdisclosure, there is provided a cable-type secondary battery,comprising: an inner electrode; a separation layer surrounding the outersurface of the inner electrode to prevent a short circuit betweenelectrodes; and an outer electrode spirally wound to surround the outersurface of the separation layer, wherein at least one of the innerelectrode and the outer anode is formed from the above-mentionedelectrode for a secondary battery according to the present disclosure.

The outer electrode may be in the form of a uniaxially extended strip.

The outer electrode may be spirally wound so that it is not overlappedin its width or overlapped in its width.

The inner electrode may be a hollow structure whose central part isempty.

The inner electrode may comprise one or more electrodes for a secondarybattery, the electrodes being spirally wound.

Also, the inner electrode may be provided with a core of inner currentcollector, a core for supplying lithium ions, which comprises anelectrolyte, or a filling core therein.

The core for supplying lithium ions may further comprise a gel polymerelectrolyte and a support, or may further comprise a liquid electrolyteand a porous carrier.

The electrolyte which is used in the core for supplying lithium ions maybe selected from a non-aqueous electrolyte solution using ethylenecarbonate (EC), propylene carbonate (PC), butylenes carbonate (BC),vinylene carbonate (VC), diethyl carbonate (DEC), dimethyl carbonate(DMC), ethyl methyl carbonate (EMC), methyl formate (MF),γ-butyrolactone (γ-BL), sulfolane, methyl acetate (MA) or methylpropionate (MP); a gel polymer electrolyte using PEO, PVdF, PVdF-HFP,PMMA, PAN, or PVAc; and a solid electrolyte using PEO, polypropyleneoxide (PPO), polyether imine (PEI), polyethylene sulphide (PES), orpolyvinyl acetate (PVAc).

The electrolyte may further comprise a lithium salt which may beselected from LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆,LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li,(CF₃SO₂)₂NLi, lithium chloroborate, lower aliphatic lithium carbonate,lithium tetraphenylborate, and a mixture thereof.

The inner electrode may be an anode or a cathode, and the outerelectrode may be a cathode or an anode corresponding to the innerelectrode.

Meanwhile, the separation layer may be an electrolyte layer or aseparator.

The electrolyte layer may comprise an electrolyte selected from a gelpolymer electrolyte using PEO, PVdF, PMMA, PVdF-HFP, PAN, or PVAc; and asolid electrolyte using PEO, polypropylene oxide (PPO), polyether imine(PEI), polyethylene sulphide (PES), or polyvinyl acetate (PVAc).

The electrolyte layer may further comprise a lithium salt, which may beselected from the group consisting of LiCl, LiBr, LiI, LiClO₄, LiBF₄,LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li,CF₃SO₃Li, (CF₃SO₂)₂NLi, lithium chloroborate, lower aliphatic lithiumcarbonate, lithium tetraphenylborate, and a mixture thereof.

The separator may be a porous polymer substrate made of apolyolefin-based polymer selected from the group consisting of ethylenehomopolymers, propylene homopolymers, ethylene-butene copolymers,ethylene-hexene copolymers, and ethylene-methacrylate copolymers; aporous polymer substrate made of a polymer selected from the groupconsisting of polyesters, polyacetals, polyamides, polycarbonates,polyimides, polyether ether ketones, polyether sulfones, polyphenyleneoxides, polyphenylene sulfides and polyethylene naphthalates; a poroussubstrate made of a mixture of inorganic particles and a binder polymer;or a separator having a porous coating layer formed on at least onesurface of the porous polymer substrate and comprising inorganicparticles and a binder polymer.

Further, in accordance with yet still another aspect of the presentinvention, there is provided a cable-type secondary battery, comprising:a core for supplying lithium ions, which comprise an electrolyte; aninner electrode surrounding the outer surface of the core for supplyinglithium ions and comprising a current collector and an electrode activematerial layer; a separation layer surrounding the outer surface of theinner electrode to prevent a short circuit between electrodes; and anouter electrode spirally wound to surround the outer surface of theseparation layer and comprising a current collector and an electrodeactive material layer, wherein at least one of the inner electrode andthe outer anode is formed from the above-mentioned electrode for asecondary battery according to the present disclosure.

Furthermore, in accordance with yet still another aspect of the presentinvention, there is provided a cable-type secondary battery, comprising:two or more inner electrodes arranged in parallel to each other; aseparation layer surrounding the outer surface of the inner electrodesto prevent a short circuit between electrodes; and an outer electrodespirally wound to surround the outer surface of the separation layer,wherein at least one of the inner electrode and the outer anode isformed from the above-mentioned electrode for a secondary batteryaccording to the present disclosure.

Further, in accordance with yet still another aspect of the presentinvention, there is provided a cable-type secondary battery, comprising:two or more cores for supplying lithium ions, which comprise anelectrolyte; two or more inner electrodes arranged in parallel to eachother, each inner electrode surrounding the outer surface of each corefor supplying lithium ions and comprising a current collector and anelectrode active material layer; a separation layer surrounding theouter surface of the inner electrodes to prevent a short circuit betweenelectrodes; and an outer electrode spirally wound to surround the outersurface of the separation layer and comprising a current collector andan electrode active material layer, wherein at least one of the innerelectrode and the outer anode is formed from the above-mentionedelectrode for a secondary battery according to the present disclosure.

The inner electrode may comprise one or more electrodes for a secondarybattery, the electrodes being spirally wound.

Thus, the sheet-form electrode for a secondary battery according to thepresent disclosure has supporting layers on at least one surfacesthereof to exhibit surprisingly improved flexibility.

The supporting layers act as a buffer when intense external forces areapplied to the electrode, e.g., during the complete folding of theelectrode, to reduce crack generation in an electrode active materiallayer even though the amount of a binder in the electrode activematerial layer is not raised. Thereby, the release of the electrodeactive material layer from a current collector can be prevented.

Accordingly, the sheet-form electrode can prevent a decrease in batterycapacity and can improve the cycle life characteristic of batteries.

Also, the sheet-form electrode has a conductive layer on the top surfaceof its electrode active material layer to provide an increasedconductivity, and the pore structure of the conductive layer allows thefree inflow of an electrolyte solution.

Further, the sheet-form electrode has a porous supporting layer to allowgood introduction of an electrolyte solution in an electrode activematerial layer, and also the electrolyte solution can be impregnatedinto the pores of the porous supporting layer to inhibit a resistancerise in the battery, thereby preventing the deterioration of batteryperformances.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and aspects of the present disclosure will become apparentfrom the following descriptions of the embodiments with reference to theaccompanying drawings in which:

FIG. 1 shows a cross-section of a sheet-form electrode for a secondarybattery according to one embodiment of the present disclosure.

FIG. 2 shows a cross-section of a sheet-form electrode for a secondarybattery according to another embodiment of the present disclosure.

FIG. 3 schematically shows a method of preparing a sheet-form electrodefor a secondary battery according to one embodiment of the presentdisclosure.

FIG. 4 shows a surface of a mesh-form current collector according to oneembodiment of the present disclosure.

FIG. 5 schematically shows a surface of a current collector having aplurality of recesses, according to one embodiment of the presentdisclosure.

FIG. 6 schematically shows a surface of a current collector having aplurality of recesses, according to another embodiment of the presentdisclosure.

FIG. 7 is a photograph showing a sheet-form electrode for a secondarybattery prepared by one embodiment of the present disclosure.

FIG. 8 schematically shows a sheet-form inner electrode being wound onthe outer surface of a core for supplying lithium ions in the cable-typesecondary battery of the present disclosure.

FIG. 9 is an exploded perspective view schematically showing the insideof a cable-type secondary battery according to one embodiment of thepresent disclosure.

FIG. 10 shows a cross-section of a cable-type secondary battery having aplurality of inner electrodes according to the present disclosure.

FIG. 11 is a photograph showing a sheet-form electrode for a secondarybattery prepared in the Example of the present disclosure, after theelectrode is folded in half.

FIG. 12 is a photograph showing a sheet-form electrode for a secondarybattery prepared in Comparative Example 1 of the present disclosure,after the electrode is folded in half.

FIG. 13 is a graph showing the life characteristics of coin-type halfcells prepared in the Example and the Comparative Examples of thepresent disclosure.

<Explanation of Reference Numerals> 10: Current collector 20: Electrodeactive material layer 30: Conductive layer 30′: Conductivematerial-containing slurry 40: First supporting layer 50: Secondsupporting layer 60: Coating blade 100, 200: Cable-type secondarybattery 110, 210: Core for supplying lithium ions 120, 220: Innercurrent collector 130, 230: Inner electrode active material layer 140,240:Conductive layer 150, 250: First supporting layer 160, 260: Secondsupporting layer 170, 270: Separation layer 180, 280: Outer electrodeactive material layer 190, 290: Outer current collector 195, 295:Protection coating

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Priorto the description, it should be understood that the terms used in thespecification and the appended claims should not be construed as limitedto general and dictionary meanings, but interpreted based on themeanings and concepts corresponding to technical aspects of the presentdisclosure on the basis of the principle that the inventor is allowed todefine terms appropriately for the best explanation.

Therefore, the description proposed herein is just a preferable examplefor the purpose of illustrations only, not intended to limit the scopeof the disclosure, so it should be understood that other equivalents andmodifications could be made thereto without departing from the spiritand scope of the disclosure.

FIGS. 1 and 2 show a cross-section of a sheet-form electrode for asecondary battery according to one embodiment of the present disclosure,and FIG. 3 schematically shows a preferred method of preparing asheet-form electrode for a secondary battery according to one embodimentof the present disclosure.

Referring FIGS. 1 to 3, a sheet-form electrode for a secondary batteryaccording to the present disclosure comprises a current collector 10; anelectrode active material layer 20 formed on one surface of the currentcollector 10; a conductive layer 30 formed on the electrode activematerial layer 20 and comprising a conductive material and a binder; anda first porous supporting layer 40 formed on the conductive layer 30.

The sheet-form electrode may further comprise a second supporting layer50 formed on another surface of the current collector 10.

In order for a battery to have flexibility, electrodes used in thebattery should have sufficient flexibility. However, in the case ofconventional cable-type batteries being one example of flexiblebatteries, an electrode active material layer is apt to be released bystress due to external force for the shape variation, or by its rapidvolume expansion during charging and discharging processes when ahigh-capacity anode active material containing Si, Sn or the like isused. Such a release of the electrode active material layer reducesbattery capacity and deteriorates cycle life characteristics. As anattempt for overcoming this problem, the amount of a binder in theelectrode active material layer has been raised to provide flexibilityduring bending or twisting.

However, the increase of a binder amount in the electrode activematerial layer causes an electrode resistance rise to deterioratebattery performances. Also, when intense external forces are applied,for example, in the case that electrodes are completely folded, therelease of the electrode active material layer cannot be prevented eventhough the amount of a binder becomes increased. Therefore, this methodis insufficient to solve such problems.

For the purpose of overcoming the above-mentioned problems, the presentinventors have designed the electrode for a secondary battery in theform of a sheet by comprising the first supporting layer 40 formed onthe outer surface thereof and the second supporting layer 50 which maybe further formed on another surface of the current collector 10.

That is, even if the electrode is applied by external forces duringbending or twisting, the first supporting layer 40 having porosity actsas a buffer capable of mitigating the external forces applied to theelectrode active material layer 20, to prevent the release of theelectrode active material layer 20, thereby improving the flexibility ofthe electrode. Also, the second supporting layer 50 which may be furtherformed can inhibit a short circuit of the current collector 10, therebymore improving the flexibility of the electrode.

Furthermore, the electrode of the present disclosure comprises aconductive layer 30 as an adhesive for adhering the first poroussupporting layer 40 with the electrode active material layer to beintegrated with each other, the conductive layer 30 being obtained bydrying a slurry containing a conductive material and a binder.

If a general binder is used as the adhesive, it acts as a resistant ofthe electrode to deteriorate battery performances. In contrast, theconductive layer 30 can improve the conductivity of the electrodewithout such a problem.

Hereinafter, a method of preparing the electrode for a secondary batterywill be explained with reference to FIGS. 1 to 3. Although FIG. 3 showsthat a conductive material layer is formed after the second supportinglayer 50 is pre-formed, this case is just one embodiment of the presentdisclosure. As will be described below, a conductive material layer maybe formed before the second supporting layer 50 is formed.

First, a slurry containing an electrode active material is applied onone surface of the current collector 10, followed by drying, to form anelectrode active material layer (S1).

The current collector 10 may be made of stainless steel, aluminum,nickel, titanium, sintered carbon, or copper; stainless steel treatedwith carbon, nickel, titanium or silver on the surface thereof; analuminum-cadmium alloy; a non-conductive polymer treated with aconductive material on the surface thereof; a conductive polymer; ametal paste comprising metal powders of Ni, Al, Au, Ag, Pd/Ag, Cr, Ta,Cu, Ba or ITO; or a carbon paste comprising carbon powders of graphite,carbon black or carbon nanotube.

As mentioned above, when secondary batteries are subject to externalforces by bending or twisting, an electrode active material layer may bereleased from a current collector. For this reason, large amounts ofbinder components are used in the electrode active material layer so asto provide flexibility in electrodes. However, large amounts of bindermay be easily peeled off owing to swelling by an electrolyte solution,thereby deteriorating battery performances.

Accordingly, for the purpose of improving adhesiveness between anelectrode active material layer and a current collector, the currentcollector 10 may further comprise a primer coating layer consisting of aconductive material and a binder. The conductive material and the binderused in the primer coating layer may be the same as those used in theformation of a conductive material-coating layer, which will bedescribed below.

Also, referring to FIGS. 4 to 6, the current collector may be in theform of a mesh, and may have a plurality of recesses on at least onesurface thereof so as to more increase its surface area. The recessesmay be continuously patterned or intermittently patterned. That is,continuous patterned recesses may be formed with spacing apart with eachother in the longitudinal direction, or a plurality of holes may beformed in the form of intermittent patterns. The plurality of holes maybe a circular or polygonal shape.

In the present disclosure, when the electrode for a secondary battery isused as an anode, the electrode active material layer may comprise anactive material selected from the group consisting of natural graphite,artificial graphite, or carbonaceous material; lithium-titanium complexoxide (LTO), and metals (Me) including Si, Sn, Li, Zn, Mg, Cd, Ce, Niand Fe; alloys of the metals; an oxide (MeOx) of the metals; a complexof the metals and carbon; and a mixture thereof, and when the electrodefor a secondary battery is used as a cathode, the electrode activematerial layer may comprise an active material selected from the groupconsisting of LiCoO₂, LiNiO₂, LiMn₂O₄, LiCoPO₄, LiFePO₄, LiNiMnCoO₂,LiNi_(1-x-y-z)Co_(x)M1_(y)M2_(z)O₂ (wherein M1 and M2 are eachindependently selected from the group consisting of Al, Ni, Co, Fe, Mn,V, Cr, Ti, W, Ta, Mg and Mo, and x, y and z are each independently anatomic fraction of oxide-forming elements, in which 0≦x<0.5, 0≦y<0.5,0≦z<0.5, and x+y+z≦1), and a mixture thereof.

Then, a slurry (30′) containing a conductive material and a binder isapplied on the electrode active material layer 20 (S2).

The conductive material-containing slurry (30′) is used as an adhesive.If only a general adhesive is used as the adhesive, it fails to formpores, making it difficult for an electrolyte solution to be introducedin the electrode active material layer, and therefore acts as aresistant of the electrode to deteriorate battery performances.

The conductive material-containing slurry (30′) forms a conductive layer30 later, and it is obtained by mixing the conductive material and thebinder in a weight ratio of 1:10 to 8:10.

The conductive material may comprise any one selected from the groupconsisting of carbon black, acetylene black, ketjen black, carbon fiber,carbon nanotube, graphene and a mixture thereof.

The binder may be selected from the group consisting of polyvinylidenefluoride (PVDF), polyvinylidene fluoride-co-hexafluoro propylene,polyvinylidene fluoride-co-trichloroethylene, polybutyl acrylate,polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone,polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide,polyarylate, cellulose acetate, cellulose acetate butyrate, celluloseacetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol,cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methylcellulose, styrene-butadiene rubber, acrylonitrile-styrene-butadienecopolymer, polyimide and a mixture thereof.

Subsequently, a first porous supporting layer 40 is formed on theapplied conductive material-containing slurry (30′).

Meanwhile, the first supporting layer 40 may be a mesh-form porousmembrane or a non-woven fabric. Such a porous structure allows goodintroduction of an electrolyte solution in the electrode active materiallayer 20, and also the first supporting layer 40 itself has superiorimpregnation of the electrolyte solution to provide good ionicconductivity, thereby preventing an electrode resistance rise andeventually preventing the deterioration of battery performances.

The first supporting layer 40 may be made of any one selected from thegroup consisting of high-density polyethylene, low-density polyethylene,linear low-density polyethylene, ultra-high molecular weightpolyethylene, polypropylene, polyethylene terephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate,polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide,polyphenylene sulfide, polyethylene naphthalate, and a mixture thereof.

Also, the first supporting layer 40 may further comprise a conductivematerial-coating layer having a conductive material and a binderthereon. The conductive material-coating layer functions to improve theconductivity of an electrode active material layer and reduce electroderesistance, thereby preventing the deterioration of batteryperformances.

The conductive material and the binder used in the conductivematerial-coating layer may be the same as those used in the conductivematerial-containing slurry which is mentioned above.

Such conductive material-coating layer is more favorable when applied ina cathode because a cathode active material layer has low conductivityto intensify performance deterioration due to electrode resistance rise,than in an anode whose active material layer has relatively goodconductivity and thus is not largely affected by the conductivematerial-coating layer to exhibit performances similar to conventionalanodes.

In the conductive material-coating layer, the conductive material andthe binder may be present in a weight ratio of 80:20 to 99:1. The use oflarge amounts of binder may induce a severe rise in electroderesistance. Therefore, when such a numerical range is satisfied,electrode resistance can be prevented from its severe rise. Also, asmentioned above, since the first supporting layer acts as a buffer whichcan prevent the release of an electrode active material layer, electrodeflexibility is not largely affected by the use of the binder in arelative small amount.

Subsequently, the resultant obtained in step (S3) is compressed to forma conductive layer 30 which is adhered between the electrode activematerial layer 20 and the first supporting layer 40 to be integratedwith each other (S4).

The conductive layer 30 improves the conductivity of the electrode toprevent the deterioration of battery performances.

The conductive layer 30 may have a porous structure for goodintroduction of an electrolyte solution in an electrode active materiallayer, and have a pore size of 0.01 to 5 μm and a porosity of 5 to 70%.

FIG. 7 is a photograph showing a sheet-form electrode for a secondarybattery prepared by one embodiment of the present disclosure.

Meanwhile, if the conductive layer 30 is formed by coating theconductive material-containing slurry (30′) on one surface of theelectrode active material layer 20, followed by drying, and then thefirst supporting layer 40 is formed by lamination thereon, the bindercomponent in the slurry (30′) for adhering the electrode active materiallayer 20 with the first supporting layer 40 may be cured, making itdifficult to maintain strong adhesion between such two layers.

Also, unlike the preferred preparation method of the present disclosurewhich uses the first porous supporting layer prepared in advance, if aporous supporting layer is formed by coating a polymer solution on theconductive layer, such a porous supporting layer formed by coating thepolymer solution has poor mechanical properties than those of the firstporous supporting layer of the prevent disclosure, thereby failing toeffectively prevent the release of the electrode active material layer.

In contrast, according to the preferred preparation method of thepresent disclosure, in the case that the first supporting layer 40 isplaced on the top of the applied conductive material-containing slurry(30′) before the binder component is cured, and then these are togethercoated by means of a coating blade 60, thereby forming the conductivelayer 30 adhered between the electrode active material layer 20 and thefirst supporting layer 40 to be integrated with each other.

Meanwhile, before the step of (S1) or after the step of (S4), the secondsupporting layer 50 may be further formed on another surface of thecurrent collector by compression. The second supporting layer 50 caninhibit a short circuit of the current collector 10, thereby moreimproving the flexibility of the current collector 10.

The second supporting layer 50 may be a polymer film which may be madeof any one selected from the group consisting of polyolefin, polyester,polyimide, polyamide and a mixture thereof.

In addition, the present disclosure provides a secondary batterycomprising a cathode, an anode, a separator interposed between thecathode and the anode, and a non-aqueous electrolyte solution, whereinat least one of the cathode and the anode is the above-mentionedelectrode for a secondary battery.

The secondary battery of the present disclosure may be in the generalform of stacking, winding or stacking/folding, and also it may be in theparticular form of cable type.

Meanwhile, a cable-type secondary battery according to the presentdisclosure comprises an inner electrode; a separation layer surroundingthe outer surface of the inner electrode to prevent a short circuitbetween electrodes; and an outer electrode spirally wound to surroundthe outer surface of the separation layer, wherein at least one of theinner electrode and the outer anode is formed from the above-mentionedelectrode for a secondary battery according to the present disclosure.

The term ‘spirally’ used herein refers to represent a helix shape thatturns around at a certain area while moving, including general springforms.

In the present disclosure, the outer electrode may be in the form of auniaxially extended strip.

Also, the outer electrode may be spirally wound so that it is notoverlapped in its width or overlapped in its width. For example, inorder to prevent the deterioration of battery performances, the outerelectrode may be spirally wound with space within the double length ofits width so that it is not overlapped.

Alternatively, the outer electrode may be spirally wound whileoverlapping in its width. In this case, in order to inhibit an excessiveresistance rise within the battery, the sheet-form outer electrode maybe spirally wound so that the width of its overlapped part may be within0.9 folds of the width of the sheet-form outer electrode itself.

The inner electrode may be a hollow structure whose central part isempty.

The inner electrode may comprise one or more electrodes for a secondarybattery, the electrodes being spirally wound.

Also, the inner electrode may be provided with a core of inner currentcollector therein.

The core of inner current collector may be made of carbon nanotube,stainless steel, aluminum, nickel, titanium, sintered carbon, or copper;stainless steel treated with carbon, nickel, titanium or silver on thesurface thereof; an aluminum-cadmium alloy; a non-conductive polymertreated with a conductive material on the surface thereof; a conductivepolymer.

Alternatively, the inner electrode may be provided with a core forsupplying lithium ions, which comprises an electrolyte therein.

The core for supplying lithium ions may comprise a gel polymerelectrolyte and a support.

Also, the core for supplying lithium ions may comprise a liquidelectrolyte and a porous carrier.

Alternatively, the inner electrode may be provided with a filling coretherein.

The filling core may be made of several materials for improving variousperformances of cable-type batteries, for example polymer resins, rubberand inorganics, besides materials forming the core of inner currentcollector and the core for supplying lithium ions, and also may havevarious forms including wire, fiber, powder, mesh and foam.

Meanwhile, FIG. 8 schematically shows a cable-type secondary batteryaccording to one embodiment of the present disclosure in which asheet-form inner electrode is wound on the outer surface of a core 110for supplying lithium ions. The sheet-form inner electrode may beapplied in cable-type secondary batteries as shown in FIG. 8, and alsothe sheet-form inner electrode may be similarly wound on the outersurface of a separation layer.

Such a cable-type secondary battery according to one embodiment of thepresent disclosure comprises a core for supplying lithium ions, whichcomprises an electrolyte; an inner electrode surrounding the outersurface of the core for supplying lithium ions and comprising a currentcollector and an electrode active material layer; a separation layersurrounding the outer surface of the inner electrode to prevent a shortcircuit between electrodes; and an outer electrode spirally wound tosurround the outer surface of the separation layer and comprising acurrent collector and an electrode active material layer, wherein atleast one of the inner electrode and the outer anode is formed from theabove-mentioned electrode for a secondary battery according to thepresent disclosure.

The cable-type secondary battery of the present disclosure has ahorizontal cross section of a predetermined shape, a linear structure,which extends in the longitudinal direction, and flexibility, so it canfreely change in shape. The term ‘a predetermined shape’ used herein isnot limited to any particular shape, and refers to any shape that doesnot damage the nature of the present disclosure.

Among cable-type secondary batteries which can be designed by thepresent disclosure, a cable-type secondary battery 100 in which theabove-mentioned electrode for a secondary battery is used as an innerelectrode is shown in FIG. 9.

Referring to FIG. 9, the cable-type secondary battery 100 comprises acore 110 for supplying lithium ions, which comprises an electrolyte; aninner electrode surrounding the outer surface of the core 110 forsupplying lithium ions; a separation layer 170 surrounding the outersurface of the inner electrode to prevent a short circuit betweenelectrodes; and an outer electrode spirally wound to surround the outersurface of the separation layer 170 and comprising an outer currentcollector 190 and an outer electrode active material layer 180, whereinthe inner electrode comprises an inner current collector 120, an innerelectrode active material layer 130 formed on one surface of the innercurrent collector 120, a conductive layer 140 formed on the top surfaceof the inner electrode active material layer 130 and comprising aconductive material and a binder, a first porous supporting layer 150formed on the top surface of the conductive layer 140, and a secondsupporting layer 160 formed on another surface of the inner currentcollector 120.

As already mentioned above, the sheet-form electrode for a secondarybattery according to the present disclosure may be used as the outerelectrode, not the inner electrode, or may be used as both of them.

The core 110 for supplying lithium ions comprises an electrolyte whichis not particularly limited to its kinds and may be selected from anon-aqueous electrolyte solution using ethylene carbonate (EC),propylene carbonate (PC), butylenes carbonate (BC), vinylene carbonate(VC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methylcarbonate (EMC), methyl formate (MF), γ-butyrolactone (γ-BL), sulfolane,methyl acetate (MA) or methyl propionate (MP); a gel polymer electrolyteusing PEO, PVdF, PVdF-HFP, PMMA, PAN, or PVAc; and a solid electrolyteusing PEO, polypropylene oxide (PPO), polyether imine (PEI),polyethylene sulphide (PES), or polyvinyl acetate (PVAc). Also, theelectrolyte may further comprise a lithium salt which may be selectedfrom LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃,LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi,lithium chloroborate, lower aliphatic lithium carbonate, lithiumtetraphenylborate, and a mixture thereof. The core 110 for supplyinglithium ions may consist of only an electrolyte, and especially a liquidelectrolyte may be formed by using a porous carrier.

In the present disclosure, the inner electrode may be an anode or acathode, and the outer electrode may be a cathode or an anodecorresponding to the inner electrode.

Electrode active materials which may be used in the anode and thecathode are the same as those which are mentioned above.

Meanwhile, the separation layer may be an electrolyte layer or aseparator.

The electrolyte layer serving as an ion channel may be made of agel-type polymer electrolyte using PEO, PVdF, PVdF-HFP, PMMA, PAN orPVAC, or a solid electrolyte using PEO, polypropylene oxide (PPO),polyethylene imine (PEI), polyethylene sulfide (PES) or polyvinylacetate (PVAc). The matrix of the solid electrolyte is preferably formedusing a polymer or a ceramic glass as the backbone. In the case oftypical polymer electrolytes, the ions move very slowly in terms ofreaction rate, even when the ionic conductivity is satisfied. Thus, thegel-type polymer electrolyte which facilitates the movement of ions ispreferably used compared to the solid electrolyte. The gel-type polymerelectrolyte has poor mechanical properties and thus may comprise asupport to improve poor mechanical properties, and the support may be aporous-structured support or a cross-linked polymer. The electrolytelayer of the present invention can serve as a separator, and thus anadditional separator may be omitted.

In the present disclosure, the electrolyte layer may further comprise alithium salt. The lithium salt can improve an ionic conductivity andresponse time. Non-limiting examples of the lithium salt may includeLiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, lithiumchloroborate, lower aliphatic lithium carbonate, and lithiumtetraphenylborate.

Examples of the separator may include, but is not limited to, a porouspolymer substrate made of a polyolefin-based polymer selected from thegroup consisting of ethylene homopolymers, propylene homopolymers,ethylene-butene copolymers, ethylene-hexene copolymers, andethylene-methacrylate copolymers; a porous polymer substrate made of apolymer selected from the group consisting of polyesters, polyacetals,polyamides, polycarbonates, polyimides, polyether ether ketones,polyether sulfones, polyphenylene oxides, polyphenylene sulfides andpolyethylene naphthalates; a porous substrate made of a mixture ofinorganic particles and a binder polymer; or a separator having a porouscoating layer formed on at least one surface of the porous polymersubstrate and comprising inorganic particles and a binder polymer.

In the porous coating layer formed from inorganic particles and a binderpolymer, the inorganic particles are bound to each other by the binderpolymer (i.e., the binder polymer connects and immobilizes the inorganicparticles), and also the porous coating layer maintains the state ofbinding with the first supporting layer by the binder polymer, In such aporous coating layer, the inorganic particles are filled in contact witheach other, from which interstitial volumes are formed between theinorganic particles. The interstitial volumes between the inorganicparticles become empty spaces to form pores.

Among these, in order for the lithium ions of the core for supplyinglithium ions to be transferred to the outer electrode, it is preferredto use a non-woven fabric separator corresponding to the porous polymersubstrate made of a polymer selected from the group consisting ofpolyesters, polyacetals, polyamides, polycarbonates, polyimides,polyether ether ketones, polyether sulfones, polyphenylene oxides,polyphenylene sulfides and polyethylene naphthalates.

Also, the cable-type secondary battery of the present disclosure has aprotection coating 195. The protection coating 195 acts as an insulatorand is formed to surround the outer current collector, therebyprotecting the electrodes against moisture in the air and externalimpacts. The protection coating may be made of conventional polymerresins having a moisture-blocking layer. The moisture-blocking layer maybe made of aluminum or a liquid-crystalline polymer which have goodwater-blocking ability, and the polymer resins may be PET, PVC, HDPE orepoxy resins.

Meanwhile, a cable-type secondary battery according to yet still anotheraspect of the present invention comprises two or more inner electrodesarranged in parallel to each other; a separation layer surrounding theouter surface of the inner electrodes to prevent a short circuit betweenelectrodes; and an outer electrode spirally wound to surround the outersurface of the separation layer, wherein at least one of the innerelectrode and the outer anode is formed from the above-mentionedelectrode for a secondary battery according to the present disclosure.

Further, the present disclosure provides a cable-type secondary battery,comprising: two or more cores for supplying lithium ions, which comprisean electrolyte; two or more inner electrodes arranged in parallel toeach other, each inner electrode surrounding the outer surface of eachcore for supplying lithium ions and comprising a current collector andan electrode active material layer; a separation layer surrounding theouter surface of the inner electrodes to prevent a short circuit betweenelectrodes; and an outer electrode spirally wound to surround the outersurface of the separation layer and comprising a current collector andan electrode active material layer, wherein at least one of the innerelectrode and the outer anode is formed from the above-mentionedelectrode for a secondary battery according to the present disclosure.

Among cable-type secondary batteries having two or more inner electrodeswhich can be designed by the present disclosure, a cable-type secondarybattery 200 in which the above-mentioned electrode for a secondarybattery is used as an inner electrode is shown in FIG. 10.

Referring to FIG. 10, the cable-type secondary battery 200 comprises twoor more cores 210 for supplying lithium ions, which comprise anelectrolyte; two or more inner electrodes arranged in parallel to eachother, each inner electrode surrounding the outer surface of each corefor supplying lithium ions; a separation layer 270 surrounding the outersurface of the inner electrodes to prevent a short circuit betweenelectrodes; and an outer electrode spirally wound to surround the outersurface of the separation layer 270 and comprising an outer currentcollector 290 and an outer electrode active material layer 280, whereineach inner electrode comprises an inner current collector 220, an innerelectrode active material layer 230 formed on one surface of the innercurrent collector 220, a conductive layer 240 formed on the top surfaceof the inner electrode active material layer 230 and comprising aconductive material and a binder, a first porous supporting layer 250formed on the top surface of the conductive layer 240, and a secondsupporting layer 260 formed on another surface of the inner currentcollector 220.

As already mentioned above, the sheet-form electrode for a secondarybattery according to the present disclosure may be used as the outerelectrode, not the inner electrode, or may be used as both of them.

In the cable-type secondary battery 200 which has a plurality of innerelectrodes, the number of the inner electrodes can be adjusted tocontrol the loading amount of the electrode active material layers aswell as battery capacity, and a probability of short circuit can beprevented owing to the presence of multiple electrodes.

Hereinafter, the present invention will be described in detail throughspecific examples. However, the description proposed herein is just apreferable example for the purpose of illustrations only, not intendedto limit the scope of the invention, so it should be understood that theexamples are provided for a more definite explanation to an ordinaryperson skilled in the art.

EXAMPLE

(1) Preparation of Cathode

A polyethylene film was compressed on one surface of a sheet-formcurrent collector being an aluminum foil, to form a second supportinglayer.

Next, another surface of the sheet-form current collector was coatedwith a cathode active material-containing slurry obtained by dispersing80 wt % of LiCoO₂ as a cathode active material, 5 wt % of Denka black asa conductive material and 15 wt % of PVdF as a binder in NMP used as asolvent, followed by drying, to form a cathode active material layer.

Subsequently, a conductive material-containing slurry obtained by mixingDenka black and PVdF in a weight ratio of 40:60 was applied on the topsurface of the cathode active material layer, and then a PET non-wovenfabric for composing a first porous supporting layer is placed on theslurry, followed by compression, to obtain a laminate having the secondsupporting layer, the current collector, the cathode active materiallayer, a layer of the conductive material slurry and the firstsupporting layer in order. Thereby, a sheet-form cathode for a secondarybattery was prepared.

(2) Preparation of Coin-Type Half-Cell

A polyethylene separator was interposed between the sheet-form cathodeprepared in step (1) and an anode consisting of a lithium foil, toobtain an electrode assembly. The electrode assembly was put in abattery case, to which 1M LiPF₆ of non-aqueous electrolyte solution wasintroduced, the electrolyte solution being obtained by mixing ethylenecarbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:2 andadding LiPF₆ to the resulting non-aqueous solvent until theconcentration of LiPF₆ became 1M. Thereby, a coin-type half-cell wasprepared.

Comparative Example 1

(1) Preparation of Cathode

A sheet-form current collector being an aluminum foil was coated with acathode active material-containing slurry obtained by dispersing 80 wt %of LiCoO₂ as a cathode active material, 5 wt % of Denka black as aconductive material and 15 wt % of PVdF as a binder in NMP, followed bydrying, to prepared a cathode.

(2) Preparation of Coin-Type Half-Cell

The procedures of step (2) of the Example were repeated except that thecathode prepared in the above step (1) of was used, to prepare acoin-type half-cell.

Comparative Example 2

(1) Preparation of Cathode

The procedures of step (1) of the Example were repeated except that aslurring containing only a PVdF binder was used instead of theconductive material-containing slurry, to prepare a cathode.

(2) Preparation of Coin-Type Half-Cell

The procedures of step (2) of the Example were repeated except that thecathode prepared in step (1) of Comparative Example 1 was used, toprepare a coin-type half-cell.

Folding Test of Electrode

The cathodes prepared in the Example and Comparative Example 1 werefolded in half, and the appearances thereof were observed.

FIGS. 11 and 12 are photographs showing the appearances of thesheet-form electrodes prepared in the Example and Comparative Example 1,respectively, after folding them in half.

As can be seen from such photographs, the electrode of ComparativeExample 1 was broken and severely cracked, whereas the electrode of theExample was not cracked and the electrode active layer thereof was wellheld by the first supporting layer made of a PET non-woven fabric. Fromthis, the electrode of the Example was confirmed to have surprisinglyimproved flexibility.

Evaluation of Charge/Discharge Characteristics

The half-cells prepared in the Example and the Comparative Examples wereeach evaluated for their charge/discharge characteristics. The batterieswere charged with a current density of 0.5 C up to 4.25 V at constantcurrent and then continuously charged with 4.25 V at constant voltage,and the charging process was completed when the current density reached0.005 C. Then, batteries were discharged with a current density of 0.5 Cup to 3.0 V at constant current. The charging/discharging was repeated20 times under the same conditions.

FIG. 13 shows the life characteristics of coin-type half cells preparedin the Example and the Comparative Examples. The cell of the Exampleexhibited life characteristic almost similar to that of ComparativeExample 1, while the cell of Comparative Example 2 exhibited very poorperformances. In the case of Comparative Example 2 using no conductivematerial, pores were not formed, making it difficult for an electrolytesolution to be impregnated in an electrode active material, andeventually acting as a resistant to deteriorate battery performances.

APPLICABILITY TO THE INDUSTRY

The present disclosure has been described in detail. However, it shouldbe understood that the detailed description and specific examples, whileindicating preferred embodiments of the disclosure, are given by way ofillustration only, since various changes and modifications within thespirit and scope of the disclosure will become apparent to those skilledin the art from this detailed description.

What is claimed is:
 1. A sheet-form electrode for a secondary battery,comprising: a current collector; an electrode active material layerformed on one surface of the current collector; a conductive layerformed on the electrode active material layer and comprising aconductive material and a binder; and a first porous supporting layerformed on the conductive layer, wherein the sheet-form electrode isformed in a substantially helical shape defined by a general springform, in which the electrode turns around a longitudinal axis whilemoving along the longitudinal axis.
 2. The electrode for a secondarybattery according to claim 1, wherein the current collector comprisesstainless steel, aluminum, nickel, titanium, sintered carbon, or copper;stainless steel treated with carbon, nickel, titanium or silver on asurface thereof; an aluminum-cadmium alloy; a non-conductive polymertreated with a conductive material on a surface thereof; a conductivepolymer; a metal paste comprising metal powders of Ni, Al, Au, Ag,Pd/Ag, Cr, Ta, Cu, Ba or ITO; or a carbon paste comprising carbonpowders of graphite, carbon black or carbon nanotube.
 3. The electrodefor a secondary battery according to claim 1, wherein the currentcollector is in the form of a mesh.
 4. The electrode for a secondarybattery according to claim 1, wherein the current collector furthercomprises a primer coating layer consisting of a conductive material anda binder.
 5. The electrode for a secondary battery according to claim 4,wherein the conductive material of the primer coating layer comprisesany one selected from the group consisting of carbon black, acetyleneblack, ketjen black, carbon fiber, carbon nanotube, graphene and amixture thereof.
 6. The electrode for a secondary battery according toclaim 4, wherein the binder of the primer coating layer is selected fromthe group consisting of polyvinylidene fluoride (PVDF), polyvinylidenefluoride-co-hexafluoro propylene, polyvinylidenefluoride-co-trichloroethylene, polybutyl acrylate, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate,polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate,cellulose acetate, cellulose acetate butyrate, cellulose acetatepropionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol,cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methylcellulose, styrene-butadiene rubber, acrylonitrile-styrene-butadienecopolymer, polyimide and a mixture thereof.
 7. The electrode for asecondary battery according to claim 1, wherein the current collectorhas a plurality of recesses on at least one surface thereof.
 8. Theelectrode for a secondary battery according to claim 7, wherein theplurality of recesses are continuously patterned or intermittentlypatterned recesses.
 9. The electrode for a secondary battery accordingto claim 8, wherein the continuous patterned recesses are formed withspacing apart with each other in a longitudinal direction.
 10. Theelectrode for a secondary battery according to claim 8, wherein theintermittently patterned recesses are formed by a plurality of holes.11. The electrode for a secondary battery according to claim 10, whereinthe plurality of holes are each of a circular or polygonal shape. 12.The electrode for a secondary battery according to claim 1, wherein thefirst supporting layer is a mesh-form porous membrane or a non-wovenfabric.
 13. The electrode for a secondary battery according to claim 1,wherein the first supporting layer is made of any one selected from thegroup consisting of high-density polyethylene, low-density polyethylene,linear low-density polyethylene, ultra-high molecular weightpolyethylene, polypropylene, polyethylene terephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate,polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide,polyphenylene sulfide, polyethylene naphthalate, and a mixture thereof.14. The electrode for a secondary battery according to claim 1, whereinthe first supporting layer further comprises a conductivematerial-coating layer having a conductive material and a binderthereon.
 15. The electrode for a secondary battery according to claim14, wherein the conductive material of the conductive material-coatinglayer and the binder of the conductive material-coating layer arepresent in a weight ratio of 80:20 to 99:1 in the conductivematerial-coating layer.
 16. The electrode for a secondary batteryaccording to claim 1, wherein the conductive layer is formed from amixture of the conductive material and the binder in a weight ratio of1:10 to 8:10.
 17. The electrode for a secondary battery according toclaim 1, wherein the conductive layer has a pore size of 0.01 to 5 μmand a porosity of 5 to 70%.
 18. The electrode for a secondary batteryaccording to claim 1, wherein the first porous supporting layer furthercomprises a porous coating layer comprising a mixture of inorganicparticles and a binder polymer thereon.
 19. The electrode for asecondary battery according to claim 1, which further comprises a secondsupporting layer formed on another surface of the current collector. 20.The electrode for a secondary battery according to claim 19, wherein thesecond supporting layer is a polymer film made of any one selected fromthe group consisting of polyolefin, polyester, polyimide, polyamide anda mixture thereof.
 21. The electrode for a secondary battery accordingto claim 1, wherein when the electrode for a secondary battery is usedas an anode, the electrode active material layer comprises an activematerial selected from the group consisting of natural graphite,artificial graphite, or carbonaceous material; lithium-titanium complexoxide (LTO), and metals (Me) including Si, Sn, Li, Zn, Mg, Cd, Ce, Niand Fe; alloys of the metals; an oxide (MeOx) of the metals; a complexof the metals and carbon; and a mixture thereof, and when the electrodefor a secondary battery is used as a cathode, the electrode activematerial layer comprises an active material selected from the groupconsisting of LiCoO₂, LiNiO₂, LiMn₂O₄, LiCoPO₄, LiFePO₄, LiNiMnCoO₂,LiNi_(1-x-y-z)Co_(x)M1_(y)M2_(x)O₂ (wherein M1 and M2 are eachindependently selected from the group consisting of Al, Ni, Co, Fe, Mn,V, Cr, Ti, W, Ta, Mg and Mo, and x, y and z are each independently anatomic fraction of oxide-forming elements, in which 0≦x<0.5, 0≦y<0.5,0≦z<0.5, and x+y+z≦1), and a mixture thereof.
 22. A method of preparinga sheet-form electrode for a secondary battery, comprising: (S1)applying a slurry containing an electrode active material on one surfaceof a current collector, followed by drying, to form an electrode activematerial layer; (S2) applying a slurry containing a conductive materialand a binder on a surface of the electrode active material layer; (S3)forming a first porous supporting layer on the applied slurry containingthe conductive material and the binder; (S4) compressing the resultantobtained in step (S3) to form a conductive layer which is adheredbetween the electrode active material layer and the first poroussupporting layer to be integrated with each other; and (S5) forming thesheet-form electrode into a substantially helical shape defined by ageneral spring form, in which the electrode turns around a longitudinalaxis while moving along the longitudinal axis.
 23. The method accordingto claim 22, wherein in the step of (S3), before the binder is cured,the first porous supporting layer is formed on the applied conductivematerial slurry.
 24. The method according to claim 22, wherein in thestep of (S4), before the binder is cured, the resultant obtained in step(S3) is compressed by a coating blade to form a conductive materiallayer which is adhered between the electrode active material layer andthe first porous supporting layer to be integrated with each other. 25.The method according to claim 22, wherein before the step of (S1) orafter the step of (S4), a second supporting layer is further formed onanother surface of the current collector by compression.
 26. A secondarybattery comprising a cathode, an anode, a separator interposed betweenthe cathode and the anode, and a non-aqueous electrolyte solution,wherein at least one of the cathode and the anode is the electrode for asecondary battery according to claim
 1. 27. The secondary batteryaccording to claim 26, which is the form of a cable.
 28. A cable-typesecondary battery comprising: two or more cores each including anelectrolyte and for supplying lithium ions; two or more inner electrodesarranged in parallel to each other, each inner electrode surrounding anouter surface of one of the two or more cores and comprising a currentcollector and an electrode active material layer; a separation layersurrounding respective outer surfaces of the inner electrodes to preventa short circuit between electrodes; and an outer electrode helicallywound to surround an outer surface of the separation layer andcomprising a current collector and an electrode active material layer,wherein at least one of the two or more inner electrodes and the outerelectrode is the electrode for a secondary battery according to claim 1.29. A cable-type secondary battery comprising: two or more cores eachincluding an electrolyte and for supplying lithium ions; two or moreinner electrodes arranged in parallel to each other, each innerelectrode surrounding an outer surface of one of the two or more coresand comprising a current collector and an electrode active materiallayer; a separation layer surrounding respective outer surfaces of theinner electrodes to prevent a short circuit between electrodes; and anouter electrode helically wound to surround an outer surface of theseparation layer and comprising a current collector and an electrodeactive material layer, wherein at least one of the two or more innerelectrodes is the electrode for a secondary battery according to claim1.